Quantitative morphology of rocks at the Mars Pathfinder landing site

摘要

We adapt and test a standard terrestrial technique of assessing morphology quantitatively for use with sites on Mars based on two-dimensional equations and particle outlines, whereby images of particles obtained remotely may yield dimensionless, numerical shape and roundness values consistent with previous knowledge of the lithology and sedimentary transport history. Our test site was the particle-rich Rock Garden at the Mars Pathfinder landing site, a location where the primary geologic processes and lithology have been determined by other means. A total of 575 particles were assessed in terms of size, sphericity, elongation, and roundness; qualitative roundness was determined for 460 particles. Particles are mostly pebble- to cobble-sized and have an average sphericity of 0.75, elongation of 0.63, and relative roundness of 0.083; 69% of particles are subangular or subrounded. Particle sphericity is unimodal, with a size-sphericity profile matching trends for terrestrial populations of similar lithology and elongations nearly identical to those for particles at Vikings 1 and 2. This matches the predictions for a site with the single overall lithology implied from other analyses. Roundness data indicate one or possibly two particle populations, representing the primary processes that likely affected particle roundness most strongly: impact and catastrophic flooding. Finally, we tested a hypothesis suggesting that darker, smaller, more angular particles comprise a population transported through impact cratering, while larger, less dark, more rounded particles are associated with catastrophic flood transport. The opposite result is seen: larger particles throughout the Rock Garden are more angular than smaller ones.